Continued efforts have been made to delineate the genetic mechanisms underlying pathogenicity of rotavirus in the newborn gnotobiotic piglet model. In these studies the VP3 and VP7 genes have been shown to play an important independent role in rotavirus growth in the gut and in induction of diarrhea. In addition, to these two outer capsid- encoding genes an additional gene (or genes) has been shown to be required for the induction of diarrhea. These findings may have implications for the development of effective reassortant rotavirus vaccines.